There are dozens of other WP installs on the same server, subject to the same ModSec rules, and none of them has a problem. This is the only WP install that has jetpack installed and it only occurs when JetPack is active. The problem is as follows:

Some actions on the post/page edit screen (so far it seems to be the ones that don't refresh the URL, like UPDATE or configure featured image) show me the following alert:
http://i.imgur.com/ULPiX.png

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1 Answer
1

This could be an issue with the After the Deadline module. When you
update a post, WordPress makes a call to the service
(service.afterthedeadline.com) to spellcheck your content, and it
seems an error occurs during the connection.

Could you try deactivating the Spelling and Grammar module

Disabling it DID fix the problem. It seems that the module does not correctly handle its job when an updated post/page content is empty. The modsec rules are not to blame (Thank you @Otto).